1. Field of the Invention
The present invention relates to an optical fiber fusion splice method in which two optical fibers each having a different mode field diameter are fusion spliced by heating using an electric discharge, and an optical fiber fusion splicer which is suitably used for the optical fiber fusion splice method.
This application is based on the Japanese Patent Applications Nos. Hei 10-271633 and Hei 11-47811, the contents of which are incorporated herein by reference.
2. Description of the Related Art
When a communication cable network is formed or a device comprising optical fibers is manufactured, two optical fibers having different mode field diameters are often fusion spliced. When the two optical fibers are simply fusion spliced using an arc discharge fusion splicer, a large splice loss results, compared with a splice between two optical fibers having the same mode field diameter.
Therefore, in order to splice optical fibers having different mode field diameters with a low splice loss, the following two splice methods have been used.
As the first splice method, a splice method has been used in which a vicinity of one end of the optical fiber having a small mode field diameter is heated using a micro-torch etc., thereby dopants contained in the core near to the end diffuse moderately and the small mode field diameter of the optical fiber is increased so as to be equal to the large mode field diameter of the other optical fiber. In this splice method, the mode field diameter at the end of both optical fibers substantially coincides. Therefore, the splice loss between the optical fibers is decreased.
However, in this first splice method, before the fusion splice is carried out using an ordinary fusion splicer, special equipment such as a micro-torch is necessary to heat the vicinity of one end of the optical fiber having a small mode field diameter. That is, another apparatus comprising a micro-torch as a heat source is necessary in addition to a fusion splicer. In addition, the problem that the work is complicated arises. Namely, a preliminary treatment using the apparatus must be performed; thereby the fusion splice steps are complicated.
As the second splice method, a splice method has been used in which two optical fibers are arranged so that the cut surfaces of the two optical fibers face each other, the ends of the optical fibers are fusion spliced by heating using an arc discharge, and then the place which was heated is additionally heated by an arc discharge. This splice method is disclosed in Japanese Patent Application, First Publication No. Hei 05-215931. This splice method is adopted to splice an optical fiber doped with erbium with an ordinary single mode optical fiber having a band of 1.55 xcexcm. In other words, when the thermal diffusion rate of the dopants contained in the core of the optical fiber having a small core diameter is large, the second splice method is adopted. The second splice method utilizes the phenomenon that when the fusion splice portion which is previously formed is heated additionally for a suitable period of time, the difference between the core diameters of two optical fibers decreases. FIG. 6 shows the relationship between the splice loss and the additional heating times when the second splice method is adopted. It is clear from FIG. 6 that the splice loss between two optical fibers can be most decreased by stopping the additional heating after a period of time.
Although, the second splice method does not need the special apparatus which is necessary for the first splice method, it has a problem that a sufficiently low splice loss can be obtained because the additional heating is carried out using an arc discharge. An arc discharge can heat only a narrow area in the longitudinal direction of the optical fiber, and the diffusion area of the core dopants is narrow in the longitudinal direction of the optical fiber. Therefore, it is difficult to decrease gradually the diffusion area of core dopant from the fusion splice portion to the non-fusion splice portion. That is, it is hard to shape the diffusion area of core dopants at the cross-sectional surface in the longitudinal direction of the optical fiber into a taper from the fusion splice portion to the non-fusion splice portion. Therefore, a sufficiently low splice loss cannot be obtained.
Furthermore, when the thermal diffusion rate of core dopants contained in the optical fiber having a large core diameter is larger than a thermal diffusion rate of core dopants contained in the optical fiber having a small core diameter, and the second splice method is adopted, the difference between the core diameters of the two optical fibers increases. Namely, when the second splice method is adopted in this situation, the undesirable effect that the splice loss increases is sometimes obtained. Specifically, as shown in FIG. 7A, when an optical fiber 10 having a thin core part 11 and an optical fiber 20 having a thick core part 21 are fusion spliced and a vicinity of the fusion splice portion is heated uniformly, not only the core dopants contained in the thin core part 11 but also the core dopants contained in the thick core part 20 diffuse. Then, as shown in FIG. 7B, a situation arises in which the difference of core diameter between the core parts 11 and 21 increases.
Therefore, the object of the present invention is to provide an optical fiber fusion splice method in which two optical fibers having different mode field diameters are fusion spliced with a low splice loss without complicated works and special equipment other than fusion splice device, and an optical fiber fusion splicer which is suitably used for the optical fiber splice method.
According to a first aspect of the present invention, the present invention provides an optical fiber fusion splice method for splicing two optical fibers having different mode field diameters comprising the steps of:
arranging a first optical fiber having a small mode field diameter and a second optical fiber having a large mode field diameter so that the cut surfaces thereof face each other,
heating by an electric discharge and fusion splicing the cut surfaces,
shifting the heating position by an electric discharge in the first optical fiber by shifting integrally the first and second optical fibers in the longitudinal direction of the optical fibers, and
additional heating the first optical fiber by an electric discharge.
According to the optical fiber fusion splice method, the additional heating by an electric discharge is carried out in the first optical fiber having a small mode field diameter, therefore, only the core dopants contained in the first optical fiber are diffused. In other words, only the mode field diameter of the first optical fiber can be gradually increased. Thereby, it is possible to correspond the mode field diameter of the first optical fiber to the mode field diameter of the second optical fiber. Furthermore, the cross-sectional shape of the mode field diameter in the longitudinal direction of the first optical fiber is tapered from the fusion splice portion to the non-fusion splice portion. Therefore, it is possible to make the splice loss of the optical fiber smaller than the splice loss of an optical fiber for which the cross-sectional surface of the mode field diameter is short and tapered, namely the splice loss of an optical fiber having a part at which the mode field diameter suddenly increases.
In particular, when the heating by an electric discharge is carried out intermittently at a plurality of positions on the first optical fiber so that the energy due to the heating by an electric discharge per fixed length is decreased in proportion to the distance that the heated position is away from the fusion splice portion, the shape of the mode field diameter of the first optical fiber can be gradually decreased from the fusion splice portion to the non-fusion splice portion. Therefore, the splice loss can be decreased.
Furthermore, when the butted cut surfaces of the first and second optical fibers are heated by an electric discharge, and additional heating is carried out continuously in the first optical fiber so that the energy due to the heating by an electric discharge per fixed length is decreased in proportion to the distance that the heated position is away from the fusion splice portion, the cross-sectional shape of the mode field diameter in the longitudinal direction of the first optical fiber is gradually tapered from the fusion splice portion to the non-fusion splice portion. Therefore, the splice loss can be more decreased.
According to a second aspect of the present invention, the present invention provides an optical fiber fusion splicer comprising a heating source, and first and second moving blocks for clamping two optical fibers and moving the two optical fibers in the longitudinal direction of the optical fibers, wherein said first moving block is mounted on the second moving block.
According to the optical fiber splicer of the present invention, only the first optical fiber clamped with the first moving block can be shifted relative to the second optical fiber clamped with the second moving block by shifting the first moving block on the second moving block. Moreover, when the second moving block is shifted, the first moving block and the second moving block are shifted integrally. Thereby, the two optical fibers positioned by the first and second moving blocks are shifted by exactly the same distance and at exactly the same time. Therefore, pressure is never added to the splice portion of the optical fibers while the two optical fibers are fusion spliced, and a low splice loss can be achieved.
According to a third aspect of the present invention, the present invention provides an optical fiber fusion splicer comprising a heating source, and first and second moving blocks for clamping two optical fibers and moving the two optical fibers in the longitudinal direction of the optical fibers, wherein said first and second moving blocks are mounted on a third moving block.
According to the optical fiber splicer of the present invention, the first and second moving blocks are integrally shifted by shifting the third moving block. Then, the two optical fibers positioned with the first and second moving blocks are shifted by exactly the same distance and at exactly the same time. Therefore, pressure is never added to the splice portion of the optical fibers while the two optical fibers are fusion spliced, and a low splice loss can be achieved.
In other words, it is possible for the two optical fibers during a fusion splice to be shifted easily by exactly the same distance and at exactly the same time. Therefore, when two optical fibers having different mode field diameters are fusion spliced by the optical fiber splice method, the heated area of the optical fiber can be enlarged by using the optical fiber splicer, thereby a fusion splice with a low splice loss can be easily achieved.